1. Field of the Invention
This invention relates to improvements to linear saturated aromatic polyesters.
2. Description of the Prior Art
High molecular weight linear thermoplastic polyesters, especially poly(ethylene terephthalate) and poly(1,4-butylene terephthalate) are well known for film and fiber production. While these polyesters undergo thermal, oxidative and hydrolytic degradation at high temperatures, by far the most serious problem is hydrolytic degradation. Many different types of additives have been described by others in an effort to overcome these problems but none have been wholly successful in overcoming the problem of hydrolytic degradation at high temperatures. Minute traces of moisture in the melt of poly(ethylene terephthalate) and other aromatic polyesters cause chain cleavage by hydrolysis with resultant loss of properties. For fiber production the moisture content must be reduced below 0.005% for satisfactory spinning operations.
It has been fairly well established that improved thermal and hydrolytic stability can be achieved by reduction of the acid content of the polyesters. Some of the types of compounds claimed in the literature as hydrolytic stabilizers for polyesters are carbodiimides, ketenimines, ketenes, and other acid reducing agents.
The major function of these additives appears to be stabilization of the end-product against hydrolytic degradation at temperatures somewhat below the melting point. Rigorous drying is required for all these additives before melt processing the polyester.
Japanese Kokai No. 7308,336 of Feb. 2, 1973 teaches monomeric cyclic acetals for acid content reduction, during melt processing.
U.S. Pat. No. 3,595,937 teaches acetals as impact modifiers for thermoplastic aromatic polyesters.
None of the known hydrolytic stabilizers allow retention of molecular weight after melt processing in the presence of small amounts of moisture.